Ink jet printing is a non-impact method for producing printed images by the deposition of ink drops in a pixel-by-pixel manner onto an image-recording receiver element in response to digital signals. For example, inkjet printing can be used for printing a colored image onto a receiver element using a stream of ink droplets directed from an inkjet printing device.
There are various methods that can be used to control the deposition of ink drops on a receiver element (substrate) to yield the desired inkjet printed image. Inkjet printing systems are generally of two types: drop-on-demand (DOD) printing systems and continuous ink jet (CIJ) printing systems. For CIJ printing systems, the direction of the stream of ink droplets is controlled electronically and causes the ink droplets to print the desired image or information on the receiver element without requiring contact between the printing device and the surface to which the ink is applied. Inkjet printers have found broad applications across markets ranging from desktop document and photographic-quality, to short run printing and industrial labeling. Objects comprising substrates to which inkjet printing is well suited include but are not limited to, containers for consumer products, currency, draft checks, envelopes, letterhead, identification cards, lottery tickets, bank cards, identification strips, labels, brochures, signage, and other well-known materials.
Drop-on-demand printing systems are widely used in home or consumer ink jet printers and slower consumer printers, both of which have been available for several decades. As the name implies, this type of inkjet printing uses a print head that ejects droplets of ink only when signaled to do so by a digital controller.
CIJ printing systems generally comprise two main components, a fluid system (including an ink reservoir) and one or more print heads. Ink can be pumped through a supply line from the ink reservoir to a manifold that distributes the ink to a plurality of orifices, typically arranged in linear array(s), under sufficient pressure to cause ink streams to issue from the orifices of the print head(s). Stimulations can be applied to the print head(s) to cause those ink streams to form streams of uniformly sized and spaced drop, which are deflected in a suitable manner, into printing or non-printing paths. Non-printing drops can be returned to the ink reservoir using a drop catcher and a return line. Thus, in contrast to DOD printing systems, CIJ printing systems involves use of a continuous stream of ink drops that are separated to discriminate between printing drops and non-printing drops. This discrimination can be accomplished in various ways known in the art. Some useful CIJ printing apparatus and print head fabrication are described for example in U.S. Pat. No. 6,588,888 (Jeanmaire et al.) and U.S. Pat. No. 6,943,037 (Anagnostopoulos et al.).
Commercially available CIJ inks are commonly aqueous dye-based inks that exhibit a number of problems. In such dye-based inks, no particles are observable under the microscope. Although there have been many recent advances in the art of dye-based ink jet inks, such inks still suffer from deficiencies such as low optical densities on coated glossy paper and poor light-fastness. When water is used as the carrier, such aqueous inks also generally suffer from poor water fastness, poor smear resistance, and uncontrolled gloss.
Most aqueous ink jet ink compositions contain non-polymeric water-miscible compounds known as “humectants” to prevent the ink jet inks from drying out in ink jet nozzles during ink jetting. Such drying out can lead to ink droplet mis-direction and printed image defects. For most aqueous ink jet ink compositions, such humectants are generally non-polymeric, nonionic hydrophilic polyols such as glycols, glycol ethers and other compounds with one or more hydroxy groups. Representative humectants are described for this purpose, for example, in U.S. Patent Application Publications 2014/0231674 (Cook) and 2014/0295152 (Brust et al.) and U.S. Pat. No. 8,142,558 (Robertson et al.). Silicon-containing polymers can be included in aqueous ink jet ink compositions to reduce wet-smear according to U.S. Pat. No. 6,124,376 (Nichols et al.). In addition, UV-curable humectants are described in U.S. Pat. No. 6,846,851 (Nakhmanovich et al.).
It has been observed that images on various papers provided using aqueous-based inkjet ink formulations containing water-soluble dyes typically exhibit lower print optical density compared to similar images prepared using offset lithographic, flexographic, or gravure printing. This is due in part to the penetration of the aqueous-based inkjet ink into the paper substrate. This problem can be more prominent using CIJ printing inks which generally have lower viscosities than printing inks used in DOD printing systems.
Thus, there is a need for aqueous particle-free inkjet ink compositions and methods for using them to provide durable and dye image-stable images having improved print optical density on various types of receiver elements (substrates).